The radio frequency (RF) spectrum is a limited commodity. Only a small portion of the spectrum can be assigned to each communications industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum. Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. The CDMA wireless communications system spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal. A signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz.
All of the wireless access terminals, including both mobile stations (e.g., cell phone) and fixed terminals, that communicate in a CDMA system transmit on the same frequency. In order for the base station to identify the wireless access terminals, each wireless access terminal is assigned a unique pseudo-random (PN) long spreading code that identifies that particular wireless access terminal to the wireless network. Typically, each long code is generated using the electronic serial number (ESN) of each mobile station or fixed terminal. The ESN for each wireless access terminal is unique to that wireless access terminal.
Similarly, each sector of a base station uses a unique short code (containing 215 bits) to identify itself to access terminals. Those familiar with the art will recognize that a sector is defined by the coverage provided by the pilot, paging and synch overhead channels transmitted by the BTS for both non-adaptive and adaptive antenna systems.
In a preferred implementation, the user data to be transmitted to a wireless access terminal is first framed, convolutionally encoded, repeated, interleaved, and encoded with the long code to form a baseband signal. The baseband signal is then separated into an in-phase (I) component and a quadrature (Q) component prior to quadrature modulation of an RF carrier and transmission. The I-component and Q-component are spread with a unique Walsh code of length M=2N uniquely assigned to each access terminal assigned to a traffic channel in the sector. The I-component is modulated by a time-offset short pseudo-random noise (I-PN) binary code sequence derived from the short code of length 215 bits. The Q-component is modulated by a time-offset short pseudo-random noise (Q-PN) binary code sequence derived from the short code of length 215 bits. In an alternate embodiment, the quadrature binary sequence may be offset by one-half of a binary chip time. Those skilled in the art will recognize that the in-phase component and the quadrature component are used for quadrature phase shift keying (QPSK) modulation of an RF carrier prior to transmission.
The maximum capacity of a base transceiver station in a CDMA wireless network is limited by the number of unique orthogonal codes (Walsh codes) that are available for assignment to traffic channels in each sector. The number of orthogonal codes available for traffic channel assignment is limited to 56-61 for IS-95; to 56-61 for Radio Configuration 1, 2 or 3 of IS-2000; and 119-125 for Radio Configuration 4 or higher in IS-2000, depending on the number of paging channels assigned. The codes allocated to traffic channels may support either voice or packet data services.
Those acquainted with the prior art will recognize that the number of simultaneous traffic channels supported over the RF links to wireless access terminals depends on the propagation environment experienced by the access terminals. For a typical good propagation mobile environment (defined in the art as Vehicular B model), the EVRC capacity supported on the forward and reverse RF links is approximately 24 Erlangs per CDMA carrier per sector in a three-sector antenna configuration. A traffic load of 24 Erlangs corresponds to 34 EVRC traffic channels with a 1% blocking probability. With an average soft handoff capacity gain of 40%, this requires 48 Walsh codes per sector on the forward link. A handoff gain of 60%, which may occur in some dense urban or highly congested areas, would require up to 54 Walsh codes.
For a wireless mobile application, the voice traffic capacity for EVRC vocoding may be as high is 65 Erlangs, or 80 traffic channels with a 1% blocking probability. For an adaptive antenna array base transceiver subsystem, a capacity increase of two to four times (i.e., 2× to 4×) translates into a requirement for up to 192 Walsh codes for 40% soft handoff gain and up to 216 Walsh codes for 60% soft handoff gain. In a non-mobile, wireless application, up to 320 Walsh codes are required. Thus, there are numerous scenarios in which the number of channels supported over the air exceeds the limit of 64 available Walsh codes for Radio Configuration 3 (or lower) or 128 available Walsh codes for Radio Configuration 4 (or greater).
Quasi-orthogonal codes have been used for increasing Walsh code availability. However, this technique results in degraded performance and lower-than-expected RF capacity due to requirements for greater Eb/No at the receiver. Another prior art method includes a segmentation of the coverage area into six sectors in non-adaptive antenna systems, which allows greater Walsh code reuse. However, the result is greater handoff transitions and increased probability of dropped calls. Those familiar with the art will recognize that doubling the number of sectors does not allow a doubling of Walsh code reuse due to the number of codes required to support soft handoff and due to added overlap regions of adjacent sector antenna patterns. However, this method is not applicable for an adaptive antenna array (AAA) base transceiver subsystem (BTS) in which multiple antennas and a baseband AAA processor module are employed per sector.
Therefore, there is a need for improved CDMA wireless networks in which the number of users per sector is not limited by the number of available Walsh codes. In particular, there is a need for a wireless CDMA adaptive antenna array base station that can more efficiently use the available Walsh codes by dynamically allocating Walsh codes in the base station sectors so that a single Walsh code may be used to communicate simultaneously with two or more wireless access terminals within the same sector. More particularly, there is a need for a CDMA wireless base station that can dynamically allocate Walsh codes in beams formed by adaptive antenna arrays of the base station so that a single Walsh code may be used to communicate simultaneously with two or more wireless access terminals in the same sector.